A supplemental type certificate (STC) is a type certificate (TC) issued when an applicant has received FAA approval to modify an aircraft from its original design.  The STC, which incorporates by reference the related TC, approves not only the modification but also how that modification affects the original design.

For complex design modifications, the Aircraft Certification Office may ask that you follow the Original Design Approval Process.

STC –

Supplemental Type Certificates (database of approved STCs in the on-line Regulatory and Guidance Library)